German Patent Application No. 10 2008 056 622 A1 shows a transfer case for distributing the torque by way of a differential to a first drive shaft and a second drive shaft. The distribution of the torque can be controlled in the transfer case by means of an auxiliary gear arrangement and automatically controllable brakes. For this purpose the transfer case has a planetary gear unit consisting of two so-called timing gears that are coupled to each other and that can be automatically controlled by means of brakes.
The planetary gear unit is coupled to a planetary stage by means of two of its shafts and is formed by a planet carrier, a set of first planet gears, a set of second planet gears, a ring gear, and a first sun gear and a second sun gear. Each first planet gear, referred to as the long planet gear, sits on a respective first planet pin, which is stationary on the planet carrier. Each second planet gear, referred to as the short planet gear, sits on a respective second planet pin, which is also stationary on the planet carrier. Each long planet gear is in mesh with a respective short planet gear and with the first sun gear. Each short planet gear is in mesh with the second sun gear. The sun gears are connected to the controllable brakes.
The planet gears and the sun gears have beveled teeth. Therefore, when torque is transferred in the course of meshing, not only are tangential and radial components of the tooth forces generated, but also axial components that are absorbed by the surrounding environment. Usually the sun gears are supported axially on friction surfaces against these forces. The friction consumes energy that is desirable in differentials on account of a certain degree of self-locking, but is undesired in timing gears. Such arrangements should move smoothly and exhibit low energy losses due to internal friction.